Secondary recovery for steeply dipping reservoirs: combined cellar and attic flooding

ABSTRACT

A method of producing oil from a steeply dipping reservoir wherein a hot aqueous fluid is injected into the reservoir which liberates gas from the oil in the reservoir. This gas then travels to the top of the reservoir where it displaces the oil at the top of the reservoir and forces it down to a production well located down dip of the top of the reservoir. The fluid travels to the bottom of the reservoir where it displaces oil upward.

United States Patent [191 Allen June 4, 1974 SECONDARY RECOVERY FORSTEEPLY DIPPING RESERVOIRS: COMBINED CELLAR AND ATTIC F LOODINGInventor: Joseph C. Allen, Bellaire, Tex.

Assignee: Texaco Inc., New York, N.Y.

Filed: Sept. 27, 1971 Appl. No.: 183,922

US. Cl. 166/272, 166/303 Int. Cl E2lb 43/00, E2lb 43/20 Field of Search166/266, 272, 279, 303,

References Cited UNITED STATES PATENTS Whorton et 211.; 166/266 X3,480,081 11/1969 Felsenthal et al 166/303 X Primary ExaminerMarvin A.Champion I Assistant Examiner lack E. Ebel Attorney, Agent, orFirmThomas H. Whaley 57 1 ABSTRACT A method of producing oil from asteeply dipping reservoir wherein a hot aqueous fluid is injected intothe reservoir which liberates gas from the oil in the reservoir. Thisgas then travels to the top of the reservoir where it displaces the oilat the top of the reservoir and forces it down to a production welllocated down dip of the top of the reservoir. The fluid travels to thebottom of the reservoir where it displaces oil upward.

3 Claims, 2 Drawing Figures PATENTEDJUN 41m 3.814.186

LIBERATED GAS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention pertains to the field of secondary recovery of oil fromsubterranean reservoirs.

2. Discussion of the Prior Art Subterranean oil bearing reservoirs arediscovered which lie at a steep angle from the horizontal. If a well isnot completed at the apex of such a reservoir, and there is no gas capto drive the oil down to production wells'some oil will necessarily beforever trapped in the portion of the reservoir above the highestproducing well since the wells will not drain the area above them. Oneway to recover this attic oil, as it is called, is to inject gas intothe formation through wells near the apex of the reservoir. This gaswill travel updip in the formation and displace the oily in the apexforcing it down to wells located lower in the structure. This is calledattic flooding. The procedure has a major drawback, however. It dependson a ready supply of gas being available for injection. The demand andmarket price of natural gas, methane and mixtures of methane and lighthydrocarbons, has precluded its use for this purpose in some instances.In other instances the supply of natural gas is not available in thefield at all. Inert gas injection has been resorted to where it is notfeasible to use natural gas; but since solution gas, the gas dissolvedin oil, is composed of hydrocarbons the solution gas vaporizes into theinert gas phase even though the pressure and temperature remainconstant. Thus, inert gas causes adverse shrinkage and viscosityincrease of the reservoir oil, both of which contribute to inefficientrecovery.

lt'is an object of this invention to provide a method of attic floodingwhich introduces free solution gas into the oil reservoir without-theneed for a supply of natural gas on the surface.

It is a further object of this invention to achieve attic flooding andsimultaneously introduce water into the reservoir which will traveldowndip and displace oil upward, which is called cellar flooding.

Briefly, the objects of this invention are accomplished by injectinginto the oil reservoir a hot aqueous fluid which is of high enoughtemperature to liberate solution gas from the reservoir oil. Thisliberated gas will travel updipto the apex of the reservoir while theinjected water travels downdip in the formation.

SUMMARY OF THE INVENTION The invention is a method whereby oil isproduced from a steeply dipping reservoir which is penetrated by atleast one well which is not completed at the very top of the reservoir,wherein a hot aqueous fluid above the bubble point temperature .of thereservoir fluids is injected into the reservoir through the well whichcauses the oil in the reservoir to liberate gas, which gas travels tothe top of the steeply dipping reservoir while the injected aqueousfluid travels to the bottom of the reservoir. The liberated gas thenforces the oil out of the topmost segment of the reservoir and down to awell or wells used for production, while the injected aqueous fluidforces the oil upward to a well or wells used for production.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a planned sketch of an oilreservoir terminating at the center against a salt dome.

FIG. 2 is a crossectional view of this same reservoir showing only well3.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of my invention isapplicable to reservoirs having certain characteristics. First, thereservoir must be steeply dipping, that is it must be at an angle suchthat gas and water will migrate updip and downdip respectively in thereservoir. Next, the reservoir must have a very small gas cap or no gascap at all since the object of this invention is to provide a method ofdisplacing oil from high in a reservoir where there is no natural methoddisplacing the oil from the apex of the reservoir. Also, the reservoirmust have solution gas present in sufficient concentrations in the oilso that it may be liberated rapidly by the hot injected water. Theprecise concentration of solution gas needed in a particular reservoirto accomplish the objects of this invention will be dependent on thephysicalproperties of the reservoir including the size and shape of theattic area above the highest production wells.

The hot aqueous fluid injected into the reservoir in the method of myinvention may be fresh water, brine, or steam for example. The fluidmust be of such temperature that it will liberate solution gas from thereservoir oil rapidly. It must, of course, be above the bubble point ofthe reservoir fluids so that gas evolution from the oil will take place.

. The method of my invention is applicable to situations where theinjection well for the hot aqueous fluid is the same as the productionwell or to situations where the injection and production wells areseparated. In the first case the hot aqueous fluid is injected into thereservoir and the gas is liberated in the reservoir. After a timeinjection is stopped and a period of time is allowed for the liberatedgas to travel upward in the reservoir and for the aqueous fluid totravel downward in the reservoir. When the gas and aqueous fluid havecleared the well bore sufficiently, production is resumed from that welland then the cycle is repeated again.

In cases where the production and injection wells are separated themethod of my invention may be continuous such that the hot aqueous fluidis injected into the injection well at the same time that oil isproduced fro the production wells.

The attached figures depict a typical embodiment of my invention. FIG. 1depicts wells situated around a salt dome. Wells 3,7, 11 and 15 areinjection wells and wells 5, 9, l3 and 17 are production wells. FIG. 2depicts a crossection of the reservoir through injection well 3. Hotwater above the bubble point of the oil in the reservoir is injectedinto the reservoir through well 3 and enters the reservoir throughopenings in the well. Gas is liberated which travels to the apex or thereservoir where it forms a secondary gas cap. The water travels downwardto the bottom of the reservoir. The generated gas cap and the waterforce the oil to the production wells where it is produced.

EXAMPLE The following example is an illustration of the tech- InjectionBefore After Pressure (PSI) 4.000 4.000 Temperature (F) 200 260 OilGravity (API) 35 35 Gas Gravity 0.7 07 /0 (Ft. /bbl) 950 820 FormationVolume LS 0.12 Gus 1.48 Liquid Total 1.60 Total Typical saturationsprior to and after water displacement are:

Oil 1 of Pure Volume Recovery (/1) Prior After Gas 0 0 Oil 80 v 40 50Water 20 fl The effect of establishing a free gas saturation byinjection of water above the bubble point temperature of the reservoirfluids is shown by the Table below of saturations prior to and afterwater invasion.

Oil "/1 of Pure Volume Recovery (71) Prior After Gas (0.8/1.6X.l2)= 6 6Oil (0.8/L6X l.48)= 74 34 57 Water V V I 20 ill.

The 6 percent gas saturation established by heat will not be produced.It will gravitate to the attic and/or remain as trapped gas behind thewater invasion. The oil recovery is increased from 50 to 57 percent.

I claim:

1. In a method whereby oil is produced from an oil column in a steeplydipping hydrocarbon reservoir which is penetrated by at least oneinjection well located downdip of at least one production well bothlocated downdip of the very top of the reservoir but in the oil columnwhich comprises liberating gas from the oil in the reservoir byinjecting a hot fluid into the reservoir through the injection wells andwithdrawing oil from the reservoir through the production wells. theimprovement which comprises liberating gas from the oil in thereservoirby injecting .the hot fluid above the bubble point temperature of thereservoir fluids through the injection wells, which gas travels to thetop of the steeply dipping reservoir forcing the oil out of the topmostsegment of the reservoir and down to the production wells while theinjected fluid travels to-the bottom of the reservoir forcing the oilupward to the production wells.

2. A method as in claim 1 wherein the hotaqueous fluid is water.

3. A method as in claim 1 wherein the hot aqueous fluid is steam.

1. In a method whereby oil is produced from an oil column in a steeplydipping hydrocarbon reservoir which is penetrated by at least oneinjection well located downdip of at least one production well bothlocated downdip of the very top of the reservoir but in the oil columnwhich comprises liberating gas from the oil in the reservoir byinjecting a hot fluid into the reservoir through the injection wells andwithdrawing oil from the reservoir through the production wells, theimprovement which comprises liberating gas from the oil in the reservoirby injecting the hot fluid above the bubble point temperature of thereservoir fluids through the injection wells, which gas travels to thetop of the steeply dipping reservoir forcing the oil out of the topmostsegment of the reservoir and down to the production wells while theinjected fluid travels to the bottom of the reservoir forcing the oilupward to the production wells.
 2. A method as in claim 1 wherein thehot aqueous fluid is water.
 3. A method as in claim 1 wherein the hotaqueous fluid is steam.